1. Field of the Invention
The present invention relates to an electromagnet for use in a solenoid-operated valve in a hydraulic system, an electromagnetic switch, or the like.
2. Description of the Prior Art
Conventional electromagnets used in solenoid-operated valves or electromagnetic switches are composed of a coil disposed around a yoke and a movable body such as a plunger inserted in the yoke and movable into out of the yoke in response to energization and de-energization of the coil for opening and closing the valve or switch. The electromagnets in the valves and switches are required to produce large magnetic forces and move the movable body with a high response.
The magnetic forces are related to the magnetic material and dimensions of the electromagnet and the cross-sectional area of the movable body which forms a magnetic circuit. Magnetic materials having higher saturation flux densities can produce greater magnetic forces. The response can be increased by passing a high exciting current momentarily to induce a magnetic flux for magnetizing the yoke rapidly for increasing the magnetic force. However, when a large magnetic flux is to be generated in a short period of time, a high eddy current flows in the magnetic material due to electromagnetic induction, and the eddy current is responsible for producing a magnetic flux in the opposite direction which cancels the magnetic flux produced by energizing the coil. Therefore, the magnetic force cannot be produced effectively, and the response cannot be increased.
The eddy current is produced in both the fixed yoke and the movable body. The magnitude of the eddy current is proportional to the rate of time-dependent change of the magnetic flux and the reciprocal of the resistivity of the magnetic material which the electromagnet is made of. Therefore, use of a magnetic material of a high resistivity can reduce the eddy current and increase the response of the movable body. Since the magnetic materials are generally electrically conductive, however, it is impossible to suppress the eddy current. Even if magnetic materials having relatively large resistivities such as a dust core, for example, are employed, difficulty arises in attaining sufficient magnetic forces with predetermined dimensions since such magnetic materials have low saturation flux densities. More specifically, to produce sufficient magnetic forces, the cross-sectional area of the magnetic path should be increased. Although the performance of the electromagnet would not be affected by substantially increasing the cross-sectional area of the magnetic path of the yoke, an increase in the cross-sectional area of the magnetic path of the movable body would result in a larger weight and a reduced deceleration of the movable body, thus making the movable body less responsive. Another way of reducing the eddy current would be to employ a lamination of silicon steel plates since silicon steel has a low electric conductivity and the lamination of silicon steel plates is effective for reducing the cross-sectional area through which the current flows and hence for increasing the electrical resistance. However, such a proposal would be disadvantageous in that it would be difficult to machine the silicon steel plates and the structure would easily be damaged or broken under external forces applied.